Sequential inactivation of Cryptosporidium parvum oocysts with ozone and free chlorine

The rate of Cryptosporidium parvum inactivation with free chlorine decreased with increasing pH in the range of 6.0–8.5, consistent with hypochlorous acid being primarily responsible for C. parvum inactivation within this pH range. The rate of sequential inactivation of C. parvum oocysts with free chlorine after pre-ozonation was characterized by an initial rapid decline in viability followed by slower kinetics with respective rates at pH 6.0 of approximately six times and twice that for free chlorine treatment without pre-ozonation under the same conditions. The greatest level of synergy was observed at pH 6; synergy decreased as pH increased until no synergy was observed at pH 8.5. Consistent with hypochlorous acid being the free chlorine species primarily responsible for C. parvum inactivation, within the pH range of 6.0–8.5 the rate of secondary inactivation with free chlorine decreased with increasing pH. Experiments designed to assess the effect of free chlorine concentration on inactivation kinetics provided conclusive evidence for the validity of the CT concept in the case of secondary inactivation of C. parvum oocysts with hypochlorous acid after ozone pre-treatment.